The present invention relates to a multi-beam exposer unit, which is used in a color printer device of a plurality of drums type, a color copy machine of a plurality of drums type, a multi-color printer, a multi-color copy machine, a monochromatic high-speed laser printer, a monochromatic high-speed digital copy machine, for scanning a plurality of beams.
For example, in an image forming device such as a color printer or a color copy machine, there are used a plurality of image forming units, and a laser exposer unit or an optical scanning device, which provides image data corresponding to color components, which are color-separated, that is, a plurality of laser beams to the image forming units.
The exposer unit has a first lens group, an optical deflector, and a second lens group. The first lens group reduces a cross-sectional beam diameter of a laser beam emitted from a semiconductor laser element to a predetermined size. The optical deflector is used to continuously deflect the laser beam reduced by the first lens group to a direction perpendicular to a direction where a recording medium is transferred. The second lens group is used to image-form the laser beam deflected by the optical deflector at a predetermined position of the recording medium. In many cases, a direction where the laser beam is deflected by the optical deflector is shown as a main scanning direction. Then, a direction where the recording medium is transferred, that is, a direction, which is perpendicular to the main scanning direction, is shown as a sub-scanning direction.
As this type of the exposer unit, the following examples are known:
Specifically, a plurality of optical scanning devices are arranged to correspond to the respective image forming sections in order to adjust to the image forming device to be applied. Also, a multi-beam exposer unit, which is formed to be capable of providing a plurality of laser beams.
In the following explanation, the direction of the rotational axis of the deflector is called as a sub-scanning direction. Also, the direction, which is perpendicular to the direction of the optical axis of the optical system and that of the rotational axis of the deflector, is called as a main scanning direction. In the image forming device, the sub-scanning direction of the optical system corresponds to the transfer direction of the transfer material. The main scanning direction shows the direction, which is perpendicular to the transfer direction in the surface of the transfer material. Also, in the following explanation, the image surface is a transfer material surface, and the image-formed surface is a surface where the beam is actually formed.
For example, there is an optical system comprising M sets of light sources, a pre-deflection optical system, serving as first optical means, and a post-deflection optical system serving as second optical means. The light sources emit Ni light beams, and at least one set of the light sources satisfies Ni.gtoreq.2. The pre-deflection optical system includes a plurality of finite focal lenses, a half mirror, which is ##EQU3## first synthetic reflection mirrors, a cylinder lens, which is M sets of optical materials, and M-1 second synthetic reflection mirrors. The finite focal lenses convert light emitted from the light source to convergent light. The half mirror puts together emission light from the respective finite focal lenses as one light beam in which the emission light is substantially overlaid on each other. Then, one light beam is synthesized to be M beam groups. The half mirror reflects a predetermined percentage of incident light and transmits a predetermined percentage of incident light. To further converge the light beams, which are synthesized to be M beam groups in the sub-scanning direction, the cylinder lens is provided with positive power having an absolute value larger than in the main scanning direction. The second synthetic reflection mirrors reflect M beam groups from the cylinder lens to be substantially overlaid on each other in the first direction.
The post-deflection optical system includes a polygon mirror, serving as one deflection means, and a pair of f.theta. lens. The polygon mirror has reflected surfaces formed to be rotatable and to deflect light in a predetermined direction. The f.theta. lens image-forms ##EQU4## beams deflected by the polygon mirror (deflection means) on a predetermined image surface to be scanned at an equal speed, and corrects influence caused by a difference between inclinations of the respective reflection surfaces of the polygon mirror.
To make the transmitting convergent light beam incident obliquely on the half mirror, the beam transmitting through the first synthetic reflection mirror generates a variation of a focal length, a spherical aberration, a coma aberration, and astigmatism.
If the thickness of the half mirror, a refractive index, and an incident angle are t, n, and u, respectively, the amount of each of the variation of a focal length, a spherical aberration, a coma aberration, and astigmatism can be shown as follows:
Variation of focal length: (t.times.(1-1/n)) PA1 Spherical aberration: Bi=-t.times.u.sup.4 .times.(n.sup.2 -1)/n.sup.3 PA1 Coma aberration: Fi=-t.times.u.sup.3 .times.(n.sup.2 -1)/n.sup.3 PA1 Astigmatism: Ci=-t.times.u.sup.2 .times.(n.sup.2 -1)/n.sup.2
In this case, the variation of the focal length can be completely canceled by increasing a length of an optical path between the finite lens and a hybrid cylinder lens by t.times.(1-1/n).
Regarding the spherical aberration, the distance between the finite lens and the cylinder lens, and the length of the optical path between the cylinder lens and the polygon mirror reflected point are suitably set such that the image surface can be moved to the center of the position where the peripheral light beam of each of the respective main scanning and sub-scanning directions intersects at the main light beam. Thereby, influence caused by the spherical aberration can be reduced.
Regarding astigmatism, the length of the optical path between the finite lens and the cylinder lens, and the length of the optical path between the cylinder length and the polygon mirror reflected point are suitably set, so that astigmatism can be completely canceled.
However, regarding the coma aberration, no correction method is proposed so far, and influence is exerted on a characteristic of the image-formation at the image surface.
In the optical system having no aberration, if the laser beam having a beam waist diameter of .omega.0 is defocused by z, the beam waist diameter of .omega. can be obtained by the following equation: EQU .omega.=.omega..sub.0 (1+(.lambda.z/(.pi..omega..sub.0.sup.2)).sup.2).sup.1/2 (A)
where .lambda. is a wavelength.
In other words, if the amount of defocus is z, the beam diameter changes from .omega.0 to .omega.. Due to the variation of the beam diameter, the thickness of the lines of the image and image density are varied.
It is assumed that f.theta. lens is formed of a plastic lens separately from the above structure.
In this case, if the lens is separated from the image surface to reduce the size of the optical system itself, the amount of defocus of the sub-scanning direction is varied by the change in temperature and humidity. In this case, the beam position of the main scanning direction is varied.